Data management in transmural care is complex. Without digital innovations like Health Information Exchange (HIE), patient information is often dispersed and inaccessible across health information systems between hospitals. The extent of information loss and consequences remain unclear. We aimed to quantify patient information availability of referred oncological patients and to assess its impact on unnecessary repeat diagnostics by observing all oncological multidisciplinary team meetings (MDTs) in a tertiary hospital. During 84 multidisciplinary team meetings, 165 patients were included. Complete patient information was provided in 17.6% (29/165, CI = 12.3–24.4) of patients. Diagnostic imaging was shared completely in 52.5% (74/141, CI = 43.9–60.9), imaging reports in 77.5% (100/129, CI = 69.2–84.2), laboratory results in 55.2% (91/165, CI = 47.2–62.8), ancillary test reports in 58.0% (29/50, CI = 43.3–71.5), and pathology reports in 60.0% (57/95, CI = 49.4–69.8). A total of 266 tests were performed additionally, with the main motivation not previously performed followed by inconclusive or insufficient quality of previous tests. Diagnostics were repeated unnecessarily in 15.8% (26/165, CI = 10.7–22.4) of patients. In conclusion, patient information was provided incompletely in majority of referrals discussed in oncological multidisciplinary team meetings and led to unnecessary repeat diagnostics in a small number of patients. Additional research is needed to determine the benefit of Health Information Exchange to improve data transfer in oncological care.